1. Field of the Invention
The invention relates to a method and an apparatus for improving mechanical properties of a magnetically activatable material, in particular for reducing ripples in metallic materials such as metal bands or metal sheets.
2. Description of the Related Art
Metal bands or metal sheets, hereinafter called materials, are finished with known treatment machines for band or sheet materials in order to achieve required material properties in the material. The finishing generally designates a process for generating changed properties in materials. These band or sheet treatment machines are known in the most varied forms and are used to bring about required material properties. They are used in band treatment or further processing lines, such as pickling lines, annealing lines, coating lines, roller mill and conditioning lines, shearing lines, stretcher and roller levelling systems or the like and combinations of different band treatment and further processing lines.
After their production, metal bands or metal sheets basically have material defects, which are characterised as edge, centre and/or longitudinal ripples. Likewise, combinations or superimpositions of edge, centre and/or longitudinal ripples are possible and lead to the most varied material defects. These material defects are generally known and appear more or less intensely as ripples in the material.
For further processing of these defective materials, these have to be flattened. Flattening describes a technical process for minimising existing ripples in the material and is generally carried out by known levelling machines or known stretcher levelling systems.
Basically, levelling machines are used for the flattening. Each levelling machine basically consists of a stable frame, in which levelling rollers are used. The levelling rollers are accommodated in a suitable device, in each case at the top and bottom inside the levelling machine. A suitable material guidance opening at the inlet and outlet of the levelling machine allows material transportation through the levelling machine and therefore through the levelling rollers. The upper and lower levelling rollers, also called a levelling roller set, are mounted together at fixed intervals and arranged one behind the other. The spacing between the inlet-side top and bottom set of levelling rollers, also known as the opening ratio, can be adjusted to transport different material thicknesses through the levelling machine and to be able to process them. The opening ratio is taken to mean the spacing between the inlet-side top and bottom levelling rollers in relation to the material thickness.
The material to be processed is continuously conveyed into the levelling machine through the material supply opening. The introduction of the material into the levelling machine and the adjustment of the opening ratio produces a force-transmitting contact between the material and the sets of levelling rollers arranged at the top and bottom. When the material is transported through the levelling machine, the material is subject to a temporally limited alternating bending stress. The transport section is thus determined by the number of predetermined levelling rollers, in which the material is forcibly guided around the individual top and bottom levelling rollers and subjected to a mechanical bending.
This alternating bending stress deforms the material at the edge regions of the surface by alternating tensile and pressure loads as a function of the opening ratio of the top and bottom sets of levelling rollers. Depending on the opening ratio, these tensile and pressure loads decrease to a greater or lesser degree in the depths of the material. A large opening ratio reduces and a small opening ratio increases the alternating bending stress of the material. If the tensile stress at the edge regions of the material surface locally reaches a limit value characteristic of the material, which is known as the yield point of the material, the material expands at these points and a stretching remains in the material, which appears as a length change.
A ripple basically present beforehand in the material is improved to a greater or lesser degree by the temporally limited alternating bending stress. This process is generally described as a levelling process.
In terms of technology, limits are set for a complete elimination of ripples in the material by the use of levelling machines. Because of the materials and material dimensions occurring today, such as the material width and material thickness, as well as the new high-strength materials to be expected in the future, the mechanical stabilities of a levelling machine are no longer adequate for targeted quality improvement of the materials, in particular to eliminate ripples in materials, in the currently known levelling machines. In addition, mechanical limits are produced on levelling machines owing to a limited opening ratio of the levelling rollers and the spacing between top and bottom sets of levelling rollers for high-strength and thin classes of material. A flattening of the materials is no longer possible here.
In general, a distinction is made between driven and non-driven levelling machines. In the driven levelling machines, the levelling rollers are driven axially by a suitable mechanical coupling to a levelling roller drive unit and the material is transported through the driven levelling rollers, which use force to clamp the material, inside the levelling machine.
In the case of the non-driven levelling machines, the material is forcibly conveyed by the levelling machine with the aid of a suitable mechanical material supply or withdrawal device, which is arranged directly in front of or behind the levelling machine.
In the case of driven and non-driven levelling machines, the material is continuously loaded with shearing and contact forces on the inlet side because of the spacing and the spatial adjustment of the levelling rollers arranged at the top and bottom. In this case, in the supply region of a levelling machine, the spacing between the top and bottom levelling rollers is significantly smaller than in the outlet region. The shearing and contact forces strongly prevailing in the supply region of a levelling machine as a result are no longer present in the outlet region of the levelling machine. Because of the declining shearing and contact forces, a constant frictional engagement cannot form between the levelling rollers and the material over the entire levelling process. An adequate frictional engagement between the levelling rollers and the material is absolutely required, however.
Because of the continuously decreasing frictional engagement in the outlet region of the levelling machine between the material and the levelling rollers, substantial surface and material defects are formed on the surface of the material in the form of scratches, furrows, or streaks, which are formed because of the decreasing or even absent frictional engagement. These surface and material defects are substantial quality deficiencies.
In particular in the case of soft materials with small material thicknesses of less than 200 μm, considerable difficulties already arise in avoiding these surface and material defects. Surface and material defects occur in particular in the levelling process when the material passes by a levelling roller arrangement as the result of an inadequate frictional engagement and the alternating bending between the material virtually disappears here and slipping of the material on the levelling rollers sharply increases due to an inadequate frictional engagement. In addition, an inadequate frictional engagement impairs the long-term, improvement required in the ripple in the material substantially. This is all the more the case as, for various material thicknesses, the rigid mechanical arrangement of the sets of levelling rollers cannot be changed and the spacing between the top and the bottom sets of levelling rollers is only varied within limits.
A treatment machine of this type for band or sheet materials is known from DE 690 03 834 T2. The material is guided through between levelling rollers and pressed together there. The force is produced magnetically, in that a main roller consists of non-magnetic material and is hollow, so that a magnet can be arranged within the main roller. The press roller cooperating therewith consists of a magnetic material, so the press roller is driven against the main roller by means of the magnet in order to produce the force, which is required there, for rolling. Thus, the use of magnets in conjunction with a roller device is known, but not for contactless material finishing. An ultrasonic device is not proposed there.
According to the current prior art, an inadequate frictional engagement between the material and the levelling rollers can only be counteracted in that the material is not subject to any contact of any kind with levelling rollers during the levelling process.